# equations imply equation

• Jul 5th 2006, 06:27 PM
malaygoel
equations imply equation
Prove that the equations
$x^2 - 3xy + 2y^2 + x - y = 0$
and $x^2 - 2xy + y^2 - 5x + 7y = 0$
imply the equation
$xy - 12x + 5y = 0$.

Keep Smiling
Malay
• Jul 6th 2006, 10:21 AM
Soroban
Hello, Malay!

I'm baffled . . . (more than usual)

Quote:

Prove that the equations:
$x^2 - 3xy + 2y^2 + x - y = 0$ and $x^2 - 2xy + y^2 - 5x + 7y = 0$
imply the equation: $xy - 12x + 5y = 0$

I thought this would be simple (perhaps imaginative) algebra,
. . but I've made absolutely no progress.

Then it occured to me that I have never done a problem like this.
Two equations imply another? . . . Do I understand what that means?
(I don't think I do . . . )

I can find the intersections of the two graphs, but that's not the mission.

The first equation represents a pair of intersecting lines.
The second is a parabola (tipped 45°).

Somehow these two imply the hyperbola: $y\:=\:\frac{12x}{x+5}$

How does that work? .I have no idea of what I'm doing . . .

• Jul 6th 2006, 11:20 AM
ThePerfectHacker
Quote:

Originally Posted by Soroban

Then it occured to me that I have never done a problem like this.
Two equations imply another? . . . Do I understand what that means?
(I don't think I do . . . )

It means, that if $x,y$ are real numbers that satisfy those equations then (implies) they satisfy a new equation.

For example, if $x+2y=1$ and $2x+y=1$ implies that they satisfy, $3x+3y=2$
• Jul 6th 2006, 01:14 PM
Soroban
Hello, TPHacker!

Thank you for the explanation.

You're saying: If $(x,y)$ satisfies the two given equations,
. . then $(x,y)$ satisfies $xy - 12x + 5y\,=\,0$

I suspected that that was an interpretation of "implies"
. . but it doesn't work.

I found three points that satisfy the two given equations:
. . $(0,0),\;(3,2),\;(5,3)$

But only $(0,0)$ satisfies: $xy - 12x + 5y\,=\,0$

• Jul 6th 2006, 01:55 PM
ThePerfectHacker
Quote:

Originally Posted by Soroban

I found three points that satisfy the two given equations:
. . $(0,0),\;(3,2),\;(5,3)$

But only $(0,0)$ satisfies: $xy - 12x + 5y\,=\,0$

This explains why you had no way of demonstrating this, because it was wrong!
• Jul 7th 2006, 01:43 AM
JakeD
Quote:

Originally Posted by Soroban
Then it occured to me that I have never done a problem like this.
Two equations imply another? . . . Do I understand what that means?
(I don't think I do . . . )

Quote:

Originally Posted by ThePerfectHacker
It means, that if $x,y$ are real numbers that satisfy those equations then (implies) they satisfy a new equation.

For example, if $x+2y=1$ and $2x+y=1$ implies that they satisfy, $3x+3y=2$

Generalizing TPH's reply, this problem can be seen one of vector spaces and linear algebra.

We are given three equations in the vector space V of equations of the form $a_1 x^2 + a_2 xy + a_3 y^2 + a_4 x + a_5 y = 0.$ V is a vector space because linear combinations of equations in V are again in V.

The three given equations

$x^2 - 3xy + 2y^2 + x - y = 0$
$x^2 - 2xy + y^2 - 5x + 7y = 0$
$xy - 12x + 5y = 0$

can be represented as coefficient vectors $[a_1, \ldots, a_5].$ Putting those 3 vectors into a matrix

$\bmatrix{
1 & -3 & 2 &1 &-1 \\
1 & 2 & 1 &-5 &7 \\
0 & 1 & 0 &-12 &5\end{bmatrix}$

the problem becomes determining whether or not the third row (equation) is a linear combination of (implied by) the first two rows (equations). The answer is no because there is no way to take a non-trivial linear combination of the first and second rows and get zeroes in both the first and third columns as in the third row. Put more abstractly, the third equation is not in the subspace spanned by the first two equations, so the third equation is not implied by the first two equations.

Back at TPH's example, the third equation is a linear combination (the sum) of the first two equations, so the third equation is implied by the first two equations.

Well, this was supposed to make the answer more obvious, but it turned out to be a long-winded way to do that! At the least I hope it makes the problem a familiar one from linear algebra.
• Jul 7th 2006, 06:33 AM
ThePerfectHacker
JakeD, I just luv your approach, but I do not understand what you are doing. Can you in steps explain what is going on. Begin by defining the vector space....
Also, does the fact
$
\bmatrix{1 & -3 & 2 &1 &-1 \\1 & 2 & 1 &-5 &7 \\0 & 1 & 0 &-12 &5\end{bmatrix}
$

Should be,
$
\bmatrix{1 & -3 & 2 &1 &-1 \\1 & -2 & 1 &-5 &7 \\0 & 1 & 0 &-12 &5\end{bmatrix}
$

Make a difference in your proof?
• Jul 8th 2006, 12:43 AM
malaygoel
it may help:
$x^3 - 3xy + 2y^2 + x - y$ $=(x-y)(x-2y+1)$

Keep Smiling
Malay
• Jul 8th 2006, 12:48 AM
malaygoel
Quote:

Originally Posted by malaygoel
Prove that the equations
$x^2 - 3xy + 2y^2 + x - y = 0$
and $x^2 - 2xy + y^2 - 5x + 7y = 0$
imply the equation
$xy - 12x + 5y = 0$.

Keep Smiling
Malay

there is one correction.
The coefficient of y in $xy - 12x + 5y = 0$ is 15 not 5.
I am sorry for it.

Keep Smiling
Malay
• Jul 8th 2006, 04:06 AM
malaygoel
Let me try once.
We have two equations
$(x-y)(x-2y+1)=0$
$x^2 - 2xy + y^2 -5x + 7y = 0$
If x=y, we have the case x=y=0 which the third equation satisfies.
$Second Case$
$x-2y+1=0$
Multiplying both sides by $(2x-y)$
We have $2x^2 - 5xy + y^2 +2x -y=0$
This equation and the second equation $(x^2 - 2xy + y^2 -5x + 7y = 0)$
produces the third equation.
Is there any other method?

Keep Smiling
Malay
• Jul 8th 2006, 05:30 AM
JakeD
Quote:

Originally Posted by ThePerfectHacker
JakeD, I just luv your approach, but I do not understand what you are doing. Can you in steps explain what is going on.

Unfortunately my approach was incorrect. The error was in these two statements.
Quote:

Originally Posted by JakeD
The problem becomes determining whether or not the third row (equation) is a linear combination of (implied by) the first two rows (equations)... Put more abstractly, the third equation is not in the subspace spanned by the first two equations, so the third equation is not implied by the first two equations.

That the third equation is not a linear combination of the first two equations does not determine that the third equation is not implied by them. I found this generally only works for linear equations. :o
• Jul 8th 2006, 07:31 AM
Soroban
Hello, all!

I found another approach . . . but hit a wall.
. . Maybe someone can carry on . . .

We want: . $\begin{pmatrix}[1]\;x^2-3xy + 2y^2 + x - y \:=\:0\\ [2]\;x^2-2xy + y^2 - 5x + 7y \:= \:0\end{pmatrix}$ $\;\Longrightarrow \;[3]\; xy - 12x + 15y \:=\:0$

$[1]\;x^2 - 3xy + 2y^2 + x - y \:=\:0\quad\Rightarrow\quad x^2$ $- 2xy + y^2 - xy + y^2 + x - y \:= \:0$

. . $\Rightarrow\quad (x-y)^2 - y(x - y) + (x - y)\:=\:0\quad\Rightarrow\quad (x-y)^2$ $- (x-y)(y-1)\:=\:0$

. . $\Rightarrow\quad(x - y)^2 \:=\:(x-y)(y-1)$ [4]

$[2]\;x^2-2xy+y^2-5x+7y\:=\:0\quad\Rightarrow\quad (x-y)^2$ $\:=\:-(5x - 7y) \:=\:0$

. . $\Rightarrow\quad (x - y)^2\:=\:5x - 7y$ [5]

Equate [4] and [5]: . $(x - y)(y - 1) \:=\:5x - 7y\quad\Rightarrow\quad xy$ $-\,6x + 8y - y^2\:=\:0$

This is not $[3]$, of course, but it's closer than anything I've found so far.

If only I could show that $y^2 \,= \,6x-7y$, but no luck yet.

Anyone? .Anyone?

• Jul 8th 2006, 08:09 AM
galactus
I will put in my 2 cents, for what it's worth. I doubt if it amounts to anything, but we'll see.

We have $x^{2}-3xy+2y^{2}+x-y=0$....[/1]

$x^{2}-2xy+y^{2}-5x+7y=0$......[2]

$xy-12x+15y=0$...........[3]

Solve [1] for x and we have $x=2y-1\,\ and\,\ x=y$

Sub $x=2y-1$ into [2] and simplify:

$y^{2}-5y+6=(y-3)(y-2)$.....[4]

Sub into [3] and simplify:

$2y^{2}-10y+12=2(y-3)(y-2)$.........[5]

Note that [5] is twice [4]

Solutions are (3,2) and (5,3).

(5,3) is the only case which produces 0 in all equations, though.

Well, except for (0,0).

Just a thought.
• Jul 8th 2006, 11:06 AM
JakeD
Quote:

Originally Posted by galactus
I will put in my 2 cents, for what it's worth. I doubt if it amounts to anything, but we'll see.

We have $x^{2}-3xy+2y^{2}+x-y=0$....[/1]

$x^{2}-2xy+y^{2}-5x+7y=0$......[2]

$xy-12x+15y=0$...........[3]

Solve [1] for x and we have $x=2y-1\,\ and\,\ x=y$

Sub $x=2y-1$ into [2] and simplify:

$y^{2}-5y+6=(y-3)(y-2)$.....[4]

Sub into [3] and simplify:

$2y^{2}-10y+12=2(y-3)(y-2)$.........[5]

Note that [5] is twice [4]

Solutions are (3,2) and (5,3).

(5,3) is the only case which produces 0 in all equations, though.

Well, except for (0,0).

Just a thought.

I put the equations into a spreadsheet and found (3,2), (5,3) and (0,0) are all solutions to all the equations. So galactus it seems your algebra is correct but the arithmetic is not.

Soroban found these these same solutions before the error in the third equation was revealed.

Quote:

Originally Posted by Soroban
Hello, TPHacker!

Thank you for the explanation.

You're saying: If $(x,y)$ satisfies the two given equations,
. . then $(x,y)$ satisfies $xy - 12x + 5y\,=\,0$

I suspected that that was an interpretation of "implies"
. . but it doesn't work.

I found three points that satisfy the two given equations:
. . $(0,0),\;(3,2),\;(5,3)$

But only $(0,0)$ satisfies: $xy - 12x + 5y\,=\,0$

• Jul 11th 2006, 04:35 AM
malaygoel
Quote:

Originally Posted by Soroban
Hello, all!

I found another approach . . . but hit a wall.
. . Maybe someone can carry on . . .

We want: . $\begin{pmatrix}[1]\;x^2-3xy + 2y^2 + x - y \:=\:0\\ [2]\;x^2-2xy + y^2 - 5x + 7y \:= \:0\end{pmatrix}$ $\;\Longrightarrow \;[3]\; xy - 12x + 15y \:=\:0$

$[1]\;x^2 - 3xy + 2y^2 + x - y \:=\:0\quad\Rightarrow\quad x^2$ $- 2xy + y^2 - xy + y^2 + x - y \:= \:0$

. . $\Rightarrow\quad (x-y)^2 - y(x - y) + (x - y)\:=\:0\quad\Rightarrow\quad (x-y)^2$ $- (x-y)(y-1)\:=\:0$

. . $\Rightarrow\quad(x - y)^2 \:=\:(x-y)(y-1)$ [4]

$[2]\;x^2-2xy+y^2-5x+7y\:=\:0\quad\Rightarrow\quad (x-y)^2$ $\:=\:-(5x - 7y) \:=\:0$

. . $\Rightarrow\quad (x - y)^2\:=\:5x - 7y$ [5]

Equate [4] and [5]: . $(x - y)(y - 1) \:=\:5x - 7y\quad\Rightarrow\quad xy$ $-\,6x + 8y - y^2\:=\:0$

This is not $[3]$, of course, but it's closer than anything I've found so far.

If only I could show that $y^2 \,= \,6x-7y$, but no luck yet.

Anyone? .Anyone?